Method of producing hydrocarbon gas

ABSTRACT

A method of producing gas from an underground formation, which is penetrated by a production well to surface, the method including allowing formation fluid comprising gas and liquid to flow into the production well at a production interval; allowing the formation fluid to separate into a gaseous component and a liquid component; producing the gaseous component through the production well to the surface; accumulating the liquid component in the production well so as to form a liquid column having, at a drainage interval of the production well, a pressure exceeding the pressure in the surrounding formation; and allowing liquid from the liquid column at the drainage interval to drain away into the surrounding formation, comprising treating the wall of the production well at the drainage interval and/or treating the formation surrounding the drainage interval so as to increase the flow rate of liquid into the surrounding formation.

FIELD OF THE INVENTION

[0001] The present invention relates to a method and system of producinggas, in particular hydrocarbon gas, from an underground formation.

BACKGROUND

[0002] In known systems for producing gas, a production well is arrangedto penetrate the gas-bearing formation. Reservoir fluid can be receivedin a production interval, e.g. through perforations in the well casingat a certain depth. The reservoir fluid often comprises liquids inaddition to the gas, in particular water.

[0003] The liquids can be present in the formation fluid when it entersthe production well, for example from a so-called high-permeabilitystreak. Liquids can also be formed by condensation on the way to thesurface in the event that the reservoir conditions (pressure,temperature) at the production interval depth are such that theformation fluid comprises vapour or liquid that is dissolved in the gas.

[0004] When the flow rate up the well is low enough, the liquid willseparate from the formation fluid under the influence of gravity, willsink to the bottom of the well where it will accumulate to form a liquidcolumn, generally a water column.

[0005] The formation of a water column is generally undesired, since theinflow of reservoir fluid will be hindered or even stopped when thewater column fully or partially overlaps the production interval. Anumber of conventional methods are used in the art to deal with theformation of a water column in a gas well.

[0006] One method is to install production tubing in the well, so-calledvelocity strings, that serve to limit the effective cross-sectional areafor the fluid produced to surface, thereby increasing the flow velocitysufficiently to prevent gas/liquid separation. Another method is to usefoaming chemicals, which lower the surface tension of separated water sothat it can be transported more easily to surface by the gas. Yetanother known method is to pump water from the water column to surface,which is also referred to as plunger lift. In a further method acompressor is used to reduce the well head pressure.

[0007] U.S. Pat. No. 5,913,363 discloses a method for downholeseparation of water from gas received from a production zone in a gaswell, using a downhole gas/water separator arranged above the productionzone, wherein the separated water is directed via packers and a pressuresensitive valve to a disposal formation.

[0008] U.S. Pat. No. 5,443,120 discloses a method for downholeseparation of water from hydrocarbons by gravity, in a portion of aninclined wellbore that is isolated by packers, and wherein separatedwater is injected into a disposal formation which has a lower pressurethan the producing formation.

[0009] U.S. Pat. No. 5,366,011 discloses a gas well wherein a specialcasing/tubing arrangement is placed. The arrangement forms an annuluscommunicating with the producing formation and has a sliding sleeve toselectively allow fluid communication between the annulus and thetubing. Water can separate from the gas in the annulus and is allowed toflow into the tubing and from there into a non-productive interval.

[0010] U.S. Pat. No. 6,336,504 discloses a method for downholeseparation and injection of water, wherein the reservoir fluid containsat least some oil, water and optionally gas, and wherein a separator isarranged at a variable position in the well so as to produce water at asufficient pressure for injection into a disposal formation.

SUMMARY OF THE INVENTION

[0011] The present invention provides a method of producing gas from anunderground formation, which underground formation is penetrated by aproduction well extending to surface, the method comprising the stepsof:

[0012] allowing formation fluid comprising gas and liquid to flow fromthe underground formation into the production well at a productioninterval;

[0013] allowing the formation fluid to separate into a gaseous componentand into a liquid component;

[0014] producing the gaseous component through the production well tothe surface

[0015] accumulating the liquid component in the production well so as toform a liquid column having, at a drainage interval of the productionwell, a pressure exceeding the pressure in the surrounding formation;and

[0016] allowing liquid from the liquid column at the drainage intervalto drain away into the surrounding formation,

[0017] wherein the step of allowing liquid from the liquid column todrain away comprises treating the wall of the production well at thedrainage interval and/or treating the formation surrounding the drainageinterval so as to increase the flow rate of liquid into the surroundingformation.

[0018] There is further provided a production well for producing gasfrom an underground formation, which well extends downwardly from theearth's surface and is arranged to penetrate the underground formation,the production well comprising:

[0019] a production interval for allowing formation fluid comprising gasand liquid to flow from the underground formation into the productionwell; and

[0020] a drainage interval,

[0021] wherein the production well is arranged so that a liquid thatseparates during normal operation from the formation fluid isaccumulated to form a liquid column covering at least partly thedrainage interval, and wherein the drainage interval is arranged so asto allow liquid from the liquid column to drain away into thesurrounding formation, by treating the wall of the production well atthe drainage interval and/or treating the formation surrounding thedrainage interval.

[0022] Liquid can be drained away into the formation by virtue of itsown weight, i.e. due to the hydrostatic pressure formed in the liquidcolumn, if there is sufficient fluid communication between the drainageinterval and the surrounding formation. This is advantageous for anumber of reasons. A first advantage is, that by applying the method alimit can be put on the height that the liquid column achieves duringnormal operation. Therefore, the barrier to inflowing formation fluidthat is formed by the liquid column is also limited. A further advantageis, that water which is contained in the reservoir fluid does not needto be produced to the surface, and can simply be disposed underground ina variety of practical situations without the need for specialreinfection facilities such as a separate reinjection well and pumps.

[0023] The drainage interval can be arranged separately underneath theproduction interval. In the event that the well comprises a longinterval that is in direct fluid communication with the surrounding gasbearing formation, and wherein during normal operation a liquid columnis formed in the well that partially overlaps this interval, the upperpart of the interval represents a production interval and the lower parta drainage interval, the boundaries being defined by the amount ofoverlap.

[0024] Allowing the liquid from the liquid column to drain away suitablycomprises treating of the wall of the well and/or treating the formationsurrounding the drainage interval so as to make it easier for liquid toflow into the surrounding formation. Treatment of the wellbore wall canbe particularly advantageous in the case when the wellbore is not cased.

[0025] Suitably, perforations are arranged in the wall of the productionwell at the drainage interval, in particular when the well is providedwith casing.

BRIEF DESCRIPTION OF THE FIGURES

[0026] The invention will now be explained by way of example in moredetail, with reference to the Figures wherein

[0027]FIG. 1 shows schematically a pressure distribution in a well witha liquid column and a gas column on top;

[0028]FIG. 2 shows calculated example curves of liquid drainage ratesQ_(l,d) as a function of the permeability-thickness product (kh)_(inj),for three liquid column heights;

[0029]FIG. 3 shows a calculated example curve of the liquid drainagetime constant τ as a function of the permeability-thickness product(kh)_(inj);

[0030]FIG. 4 shows calculated example curves of the height H as afunction of the permeability-thickness product (kh)_(inj), for threeliquid entry rates;

[0031]FIG. 5 shows schematically a first embodiment of the invention;and

[0032]FIG. 6 shows schematically a second embodiment of the invention.

DETAILED DESCRIPTION

[0033] Reference is made to FIG. 1. The Figure shows schematically thedistribution of the pressure p (units: Pa) along the depth d (units: m)of a vertical well which has a liquid column at the bottom and a gascolumn on top thereof, in a static situation such as during shut-in of agas well. The pressure at the surface (d=0) is denoted with p₀. The wellis filled with gas between the surface and the depth of the top of theliquid column, d_(l). The pressure p in this gas filled part 1 increaseslinearly with depth d, p(d)=p₀+ρ_(g)gd, wherein ρ_(g) is the density ofthe gas (kg/m³), and g is the standard gravity of the earth.

[0034] In the liquid column (reference numeral 3 in the Figure), thehydrostatic pressure p increases with depth proportional with thedensity of the liquid, p_(l) (kg/m³),p(d)=p₀+ρ_(g)gd_(t)+ρ_(l)g(d−d_(l)). When at a certain depth the well isin fluid communication with the surrounding formation, and when thepressure in the formation is lower there, liquid will drain into theformation.

[0035] In a simple model for shut-in of a gas well, the pressuredistribution as a function of depth in the gas bearing formationsurrounding the well is equal to the pressure distribution of anentirely gas-filled well when the well is closed at the top, i.e.corresponds to the pressure distribution as formed by parts 1 and 5 ofthe pressure curve in FIG. 1. In this case, when the well is providedwith drainage perforations at a depth d_(p)>d_(l), the driving force forthe drainage of a liquid column is the pressure differenceΔp=(ρ_(l)−ρ_(g))g(d_(p)−d_(l)).

[0036] In this case the drainage rate Q_(l,d) (m³/s) with which theliquid column drains away can be estimated as $\begin{matrix}{{Q_{1,d} = {{\frac{2{\pi ({kh})}_{inj}}{\mu_{1}\{ {{\ln ( \frac{r_{e}}{r_{w}} )} + S} \}} \cdot \Delta}\quad p}},} & (1)\end{matrix}$

[0037] wherein

[0038] (kh)_(inj) denotes the permeability-thickness product of theformation at the drainage interval (m₃);

[0039] μ_(l) denotes the viscosity of the liquid (Pa.s);

[0040] r_(e) is the drainage radius of the well (m);

[0041] r_(w) is the well bore radius (m);

[0042] S is the skin factor (numeral);

[0043] and Δp has been defined before.

[0044] An expression for the characteristic time for drainage of aliquid column can be derived under the assumption, that the liquidvolume dV that drains away in a differential time unit dt isproportional to the height H of the liquid column above the drainageinterval, H=(d_(p)−d_(l)). The proportionality constant is obtained fromequation (1). Integration of a differential equation thus obtainedyields a single exponential decay of the height of the liquid columnwith time, wherein the time constant τ (s) is given by $\begin{matrix}{{\tau = \frac{r_{w}^{2}\mu_{1}\{ {{\ln ( \frac{r_{e}}{r_{w}} )} + S} \}}{2({kh})_{inj}{\Delta\rho}\quad g}},} & (2)\end{matrix}$

[0045] wherein Δρ=(ρ_(l)−ρ_(g)), and wherein all other symbols have thesame meaning as defined before.

[0046] Reference is now made to FIG. 2, which shows liquid drainagerates Q_(l,d) (in m³/day) as a function of the permeability-thicknessproduct (kh)_(inj) (in millidarcy.meter). Curves are shown for threedifferent heights H of the top of the liquid column above the top of thedrainage perforations, a) H=5 m; b) H=25 m; c) H=100 m. The curves havebeen calculated on the basis of equation (1), using the followingparameters of Table 1 which have been selected for a typical gas well.TABLE 1 Quantity Value R_(e)  850 m R_(w)  0.1 m (mechanical) well skinS +5 Liquid (water) viscosity μ_(l) 0.4 mPa.s Liquid (water) densityρ_(l) 1000 kg/m³ Gas density ρ_(g)  75 kg/m³

[0047] The rate at which liquid (water) enters the well Q_(l,e) duringgas production is typically in the order of 1 . . . 4 m³/day, and isalso indicated in FIG. 2. The Figure indicates that when the well isshut in, the rate with which the water drains away is in the same orderof magnitude or larger than the water entry rate.

[0048]FIG. 3 shows the time constant τ (in days) of equation 2 as afunction of the permeability-thickness product (kh)_(inj) (inmillidarcy.meter), calculated using the parameter values of Table 1.

[0049] Equations 1 and 2 have been derived for a gas well that isshut-in, i.e. closed at the surface so that no gas is produced. When thewell is shut-in for about 5 times the time constant τ, the liquid columnabove the drainage perforations will have disappeared.

[0050] Also when the well is not shut-in, water will drain away whilereservoir fluid is received in the well in a production interval abovethe drainage interval. In a steady state situation, the liquid entryrate Q_(l,e) equals the liquid drainage rate Q_(l,d). An estimate forthe liquid column height H (m) in the steady state can be obtained byrearranging equation (1) and by substituting Q_(l,d) for Q_(l,e), giving$\begin{matrix}{{H = \frac{Q_{1,e} \cdot \mu_{1} \cdot \{ {{\ln \quad ( \frac{r_{e}}{r_{w}} )} + S} \}}{2{\pi ({kh})}_{inj}{\Delta\rho}\quad g}},} & (3)\end{matrix}$

[0051] wherein all symbols have the meaning as defined before.

[0052]FIG. 4 shows the height H calculated by using equation (3) usingthe parameters in Table 1 for three liquid entry rates, a) Q_(l,e)=1m³/day; b) Q_(l,e)=2 m³/day; c) Q_(l,e)=4 m³/day.

[0053] It will be clear that, if the liquid entry rate Q_(l,e) is largerthan a critical liquid entry rate Q_(l,e;crit), the height of the liquidcolumn would become larger than the well could accommodate for normaloperation. In that case the production of the gaseous component to thesurface can be interrupted at intervals for a time period long enough toallow sufficient liquid to drain away into the formation, e.g. 5 timesthe time constant τ. During these shut-ins the liquid column height isreduced to below a predetermined height, whereafter production can becontinued.

[0054] When the liquid entry rate is smaller than the critical liquidentry rate, the gaseous component can be produced continuously to thesurface, while liquid is allowed to drain away simultaneously withproducing the gaseous component.

[0055] The value of the critical liquid entry rate Q_(l,e;crit) dependson a number of factors such as the the liquid/gas ratio of the inflowingreservoir fluid, the well geometry, the arrangement of perforations, thedrainage characteristics, and the reservoir pressure and temperature. Itcan in principle be determined using a simulation tool.

[0056] Depending on the practical situation, there are a number of waysfor allowing the liquid to drain away at the drainage interval.

[0057] When the wellbore is left uncased at the drainage interval,draining may occur naturally once a liquid column of sufficient heightis formed.

[0058] Another suitable way is to arrange perforations in the wall ofthe production well, in particular when the well is provided withcasing.

[0059] If in a given situation the liquid entry rate is larger than thecritical liquid entry rate, the drainage rate of liquid into theformation can be increased by treating the wall of the production wellat the drainage interval and/or treating the formation surrounding thedrainage interval. This treatment makes it easier for liquid to flowinto the surrounding formation. Treatment of the wellbore wall can beparticularly advantageous in the case when the wellbore is not cased.Such a treatment normally reaches only a limited distance into theformation.

[0060] One suitable treatment of the wellbore wall is a treatment usingchemicals. For example, an acid such as hydrochloric acid can be used inorder to remove mud and fines that have precipitated at the wellborewall, thereby lowering the barrier (“skin”) for liquid to flow into theformation. The acid can be pumped downhole and will mix with the watercolumn. Acid does not remain active for long, normally shorter than 24hours.

[0061] In a suitable treatment of the formation surrounding the drainageinterval, pressure pulses are applied to the liquid column so as togenerate micro fractures deeper in the formation surrounding thedrainage interval. Pressure pulses can be provided using means known inthe art via hydraulic pulses, also referred to as acoustic pulses.

[0062] Chemical and pressure treatment can also be applied incombination, so as speed up the chemical reaction, and also in order toedge microfractures that are formed in the formation.

[0063] When the flow rate Q_(g) (m³/s) of the gas up the production wellis low enough, and consequently the flow velocity in the well, the fluidwill separate so that the liquid will sink to the bottom of the wellwhere it will accumulate to form a liquid column.

[0064] At higher gas flow rates liquid separation does not occurnaturally. An estimate for the critical gas flow Q_(g,crit), below whichseparation occurs naturally under the influence of gravity, can beobtained by the equation $\begin{matrix}{{Q_{g,{crit}} = {\frac{\pi \quad D^{2}c}{4} \cdot \lbrack \frac{( {\rho_{1} - \rho_{g}} )g\quad \sigma}{C_{D}\rho_{g}^{2}} \rbrack^{1/4}}},} & (4)\end{matrix}$

[0065] which is also known in the art as the Turner criterion. Thesymbols used in the equation have the following meaning:

[0066] D=diameter of the well (m);

[0067] c=a numerical constant, suitably in the order of 2;

[0068] ρ_(l)=density of the liquid (kg/m³);

[0069] ρ_(g)=density of the gas (kg/m³);

[0070] σ=surface tension (kg/s²);

[0071] C_(D)=drag coefficient (numerical).

[0072] A typical critical gas flow rate corresponds to a gas velocity ofapproximately 5-6 m/s.

[0073] When the gas flow rate is above the critical gas flow rate, theflow energy of the gas is sufficiently high to carry the liquid tosurface, which is also called the mist flow regime. In this event, themethod of the present invention can advantageously be used by arranginga gas/liquid separator, either in the well or on surface. This separatoris arranged so as to receive formation fluid through an inlet, and hasoutlets for at least a gaseous stream and a liquid stream. The liquidstream is then used to form the liquid column in the well. Suitableseparators for this purpose are known in the art, e.g. a cycloneseparator, a plate pack separator, a curved guiding vane separator, or amist mat.

[0074] When the formation fluid contains vapour or fluid dissolved inthe gas that condenses only when the fluid has risen to a certain depthin the well, the separator is preferably arranged above this depth.

[0075] Reference is now made to FIG. 5, showing schematically a firstembodiment of the invention. A production well 11 extends verticallydownwardly from the surface 15 and penetrates a gas-bearing formation20. The well is provided with casing (not shown), and perforations arearranged at a production interval 24 and a drainage interval 28 belowthe production interval. In the well, above the drainage interval, thereis a separator 30 having an inlet for reservoir fluid 32, an outlet forgas 34, and an outlet for liquid 36. A conduit 40 is arranged from theoutlet 36 to a position 42 below the production interval, within orabove the region wherein during normal operation the liquid column 44 isformed. The conduit 40 serves to prevent the separated liquid from beingre-entrained by the gas. Production tubing 48 provides fluidcommunication for the gas between the outlet 34 and the wellhead 50.

[0076] During normal operation, reservoir fluid comprising gas and waterflows into the well 11 at the production interval 24. The reservoirfluid rises in the well and enters the separator 30 through the inlet32. The separator separates the reservoir fluid into a componentconsisting mainly of gas, and into a liquid component. The gas componentis conducted to surface via production tubing 48. The liquid is guidedto a position below the production interval, where a liquid column 44 isformed.

[0077] The height of the liquid column above the drainage intervalperforations 28 exerts a hydrostatic pressure larger than the pressurein the formation 20 at the drainage interval. Thereby the liquid fromthe water column 44 can drain into the formation through theperforations at the drainage interval. When the liquid entry rateQ_(l,e) is smaller than the critical liquid entry rate Q_(l,e;crit), thegaseous component can be produced continuously to the surface, whileliquid is allowed to drain away simultaneously. If needed, the drainageinterval or the formation surrounding the drainage interval can betreated by one of the methods described hereinbefore, so as to allowcontinuous operation, in particular so that the flow rate of inflowingwater at the production interval 24 substantially equals the rate ofwater drained into the formation at the drainage interval 28.Alternatively, if the liquid entry rate is too large, production of gascan be stopped by closing the production tubing 48 at the wellhead 50,so as to allow more time for the liquid to drain away.

[0078] When the water has drained into the formation 20, it willpreferably sink in the formation to the level of the water gas contact55, so that the drained water is not produced back. The time scale ofthis process is determined by the vertical permeability of theformation.

[0079] It shall be clear that the separator 30 does not need to bepresent if the rate of inflowing gas is below the critical gas flowrate.

[0080] Reference is made to FIG. 6, which shows schematically anotherembodiment of the invention. Like numerals as in FIG. 5 are used torefer to the same objects. The well 60 is cased only at the top anduncased at and below the region 62.

[0081] During normal operation of this well, formation fluid comprisinggas and water enters the well 60 in the uncased part and rises. Thetotal gas flow rate is low enough so as to allow separation of waterdroplets 63 which sink to the bottom of the well where they accumulateto form a liquid column 44. The liquid column, during normal operation,extends to a level which defines the lower end of region 62.

[0082] In region 62, reservoir fluid can enter the well withouthindrance.

[0083] In the region 64, the pressure in the well due to the hydrostaticpressure of the water column is still smaller than the pressure in theformation, so that in region 64 also formation fluid can enter the wellas indicated by the arrows, be it somewhat hindered as compared toregion 62. Regions 62 and 64 form the production interval.

[0084] In the area 66 the hydrostatic pressure in the liquid column issuch that the well pressure about equals the pressure in the surroundingformation, so that virtually no fluid is exchanged between well andformation there.

[0085] In the region 68, the hydrostatic pressure is large enough sothat the water can drain into the formation. Region 68 forms thedrainage interval.

[0086] Again, the wellbore wall and/or surrounding formation can betreated to as to allow sufficient fluid to be drained away forcontinuous operation.

[0087] It shall be clear that four basic regimes for operation can bedistinguished in the operation of the method and system of the presentinvention (using the symbols as defined before):

[0088] 1. Q_(g)<Q_(g,crit) and Q_(l,e)<Q_(l,e;crit), so no gas/liquidseparator is needed, and continuous operation is possible;

[0089] 2. Q_(g)<Q_(g,crit) and Q_(l,e)>Q_(l,e;crit), so no gas/liquidseparator is needed, but discontinuous operation is required;

[0090] 3. Q_(g)>Q_(g,crit) and Q_(l,e)<Q_(l,e;crit), so a gas/liquidseparator is needed, and continuous operation is possible;

[0091] 4. Q_(g)>Q_(g,crit) and Q_(l,e)>Q_(l,e;crit), so a gas/liquidseparator is needed as well as discontinuous operation.

[0092] Instead of discontinuous operation, further treatment of thewellbore and/or formation can be performed so as to make it easier forliquid to drain into the formation, thereby increasing Q_(l,e;crit).

We claim:
 1. A method of producing gas from an underground formationpenetrated by a production well, the production well extending tosurface, the method comprising the steps of: allowing formation fluidcomprising gas and liquid to flow from the underground formation intothe production well at a production interval; allowing the formationfluid to separate into a gaseous component and into a liquid component;producing the gaseous component through the production well to thesurface; accumulating the liquid component in the production well so asto form a liquid column having, at a drainage interval of the productionwell, a pressure exceeding the pressure in the surrounding formation;and allowing liquid from the liquid column at the drainage interval todrain away into the surrounding formation, wherein the step of allowingliquid from the liquid column to drain away comprises treating the wallof the production well at the drainage interval and/or treating theformation surrounding the drainage interval so as to increase the flowrate of liquid into the surrounding formation.
 2. Method according toclaim 1, wherein the step of allowing liquid to drain away comprisesarranging perforations in the wall of the production well at thedrainage interval.
 3. The method according to claim 1, wherein the wallof the production well is treated by adding a chemically active agent tothe liquid.
 4. The method according to claim 3, wherein the chemicallyactive agent is an acid.
 5. The method according to claims 1, whereinthe surrounding formation is treated by generating fractures therein. 6.The method according to claims 1, wherein pressure pulses are applied tothe liquid column.
 7. The method according to claim 1, wherein the stepof allowing the formation fluid to separate comprises controlling theflow rate of the produced gaseous component in the production well toremain below a critical gas flow rate, such that formation fluid canseparate in the production well in the absence of a dedicated separator.8. The method according to claim 1, wherein the step of allowing theformation fluid to separate comprises admitting the formation fluid tothe inlet of a gas/liquid separator, the gas/liquid separator havingoutlets for the gaseous component and for the liquid component.
 9. Themethod according to claim 8, wherein the liquid received at the outletfor liquid of the separator is guided through a conduit to a positionbelow the production interval.
 10. The method according to claim 8,wherein the gas/liquid separator is arranged in the production well. 11.The method according to claim 10, wherein the formation fluid comprisesliquid vapour or liquid dissolved in the gas, and wherein the gas/liquidseparator in the production well is arranged above the depth, at whichduring normal operation liquid condenses from the formation fluid. 12.The method according to claim 8, wherein the separator is arranged atthe surface.
 13. The method according to claim 1, wherein, during normaloperation, the gaseous component is produced continuously to thesurface, and wherein liquid from the liquid column is allowed to drainaway simultaneously with producing the gaseous component.
 14. The methodaccording to claim 1, wherein, during normal operation, the productionof the gaseous component is at intervals interrupted for a period longenough to allow sufficient liquid from the liquid column to drain awayinto the formation, so that the liquid column height is reduced to belowa predetermined height.
 15. The method according to claim 1, wherein thegaseous component mainly consists of hydrocarbon gas, and wherein theliquid mainly consists of water.
 16. A production well for producing gasfrom an underground formation, which well extends downwardly from theearth's surface and is arranged to penetrate the underground formation,the production well comprising: a production interval for allowingformation fluid comprising gas and liquid to flow from the undergroundformation into the production well; and a drainage interval, wherein theproduction well is arranged so that a liquid that separates duringnormal operation from the formation fluid is accumulated to form aliquid column covering at least partly the drainage interval, andwherein the drainage interval is arranged so as to allow liquid from theliquid column to drain away into the surrounding formation, by treatingthe wall of the production well at the drainage interval and/or treatingthe formation surrounding the drainage interval.
 17. A production wellaccording to claim 16, further comprising a gas/liquid separator havingan inlet for formation fluid, an outlets for a separated gaseouscomponent and an outlet for a separated liquid component.